Leases (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Future minimum lease and royalty payments
Capital Leases, 2013	$ 42.2
Capital Leases, 2014	26.6
Capital Leases, 2015	11.2
Capital Leases, 2016	11.1
Capital Leases, 2017	10.9
Capital Leases, 2018 and Thereafter	23.4
Capital Leases, Total minimum lease payments	125.4
Capital Leases, Less interest	20.8
Capital Leases, Present value of minimum capital lease payments	104.6
Operating Leases, 2013	145.1
Operating Leases, 2014	130.5
Operating Leases, 2015	115.1
Operating Leases, 2016	102.3
Operating Leases, 2017	81.6
Operating Leases, 2018 and thereafter	102.9
Operating Leases, Total minimum lease payments	677.5
Coal Lease and Royalty Obligation, 2013	282.2
Coal Lease and Royalty Obligation, 2014	280.4
Coal Lease and Royalty Obligation, 2015	278.0
Coal Lease and Royalty Obligation, 2016	252.6
Coal Lease and Royalty Obligation, 2017	4.6
Coal Lease and Royalty Obligation, 2018 and Thereafter	28.3
Coal Lease and Royalty Obligation, Total minimum lease payments	1,126.1
Leases (Textuals)
Rental expense under operating leases	247.5	170.6	129.5
Property, plant, equipment and mine development assets, gross value under capital leases	162.5	177.2
Accumulated depreciation of property, plant, equipment and mine development assets under capital leases	83.3	46.6
Total royalty expenses on coal reserve leases	637.5	610.6	540.6
Initial lease term for federal leases	ten years
Minimum annual production on federal leases	1.00%
Monthly royalty percentage on federal leases for coal sales using surface mining methods	12.50%
Monthly royalty percentage on federal leases for coal production using underground mining methods	8.00%
Period to redetermine royalty rates on leased coal reserves in Arizona	every ten years
Initial term of mining leases in Australia	up to 21 years
Company's lease obligations secured by outstanding surety bonds	$ 105.3